1. Field of the Invention
The present invention refers to an ultrasonic imaging device which makes it possible to obtain an ultrasonic image with high resolving power over a wide range of imaging object region.
2. Description of the Prior Art
As an ultrasonic imaging device which has been known in the past, there is an ultrasonic imaging device of linear electronic scanning type. This device has a large number of slender quadrature vibrators arranged in a line, and the required directivity is provided by driving the vibrators. In a device of the above description, a method has been employed in which the time control of the transmission and reception system has been carried out by means of the delay lines, and an improvement in azimuthal resolution has been accomplished by converging the distributions of the ultrasonic beam and the distribution of the reception sensitivity. Moreover, in order to converge the distributions of the ultrasonic beam and the reception sensitivity over a wider range, the multi-stage focusing method has been used in which the focal point is switched in succession.
However, in a method like the above, the switching of the focal point has to be done many times in order to obtain a high resolution over a wide range in the distance direction, namely the transmission direction of the ultrasonic beam. This gives rise to problems such as the prolongation of the time required for the operation, complication in the delay line system used for converging the ultrasonic beam, and the like.
On the other hand, there is known another method, the synthetic aperture method, for obtaining high resolution. According to the synthetic aperture method, an ultrasonic beam with spread greater than a predetermined value is radiated from the vibrators to the region of the object to be imaged, and the reflected waves from the object are received by the vibrators with sensitivity distribution greater than a certain value. Then, the received signals are phase detected in order to obtain the quadrature signals relating to the object, namely, the holograms. The data relating to the reconstruction of the object is obtained by carrying out the aperture synthesis operation using the hologram signals and the kernel signals.
According to the above method, high resolution can be obtained over a wide range. However, due to the fact that small vibrators have to be used in order to obtain an ultrasonic beam and the distribution of the reception sensitivity that have spreads greater than certain values, the received signals become weak, giving rise to a reduction in the SN ratio.